It's a little difficult isn't it)) That sentence sounds indefinitely. Therefore, you'll get the follow-up question: О каком человеке? (who are you talking about?). In this case you should use the word "человечество". Mankind means человечество or человеческий род.

"этого" is the masculine/neuter version of "это", but in the genitive case. This means that if the noun in the sentence is in the genitive case, the pronoun will have to do the same. For example, "этого мальчика" means "That boy".

Not exactly. If you say "15 человек" then yes it means "15 persons", males and females. But if you say "возле дома стоял человек" it almost always means a man (male).

One more thing. "Молодой человек" is 1) very popular address to young and not so young men; 2) boyfriend.

As for addresses Russian had "товарищ" or "гражданин" in Soviet era, not popular now. "Господин/госпожа" is used officially but not yet too popular in other cases. So most popular are "молодой человек / девушка / мужчина / женщина" depending on gender and age. Some people don't like when they are addressed as "мужчина / женщина" probably due to the fact that they don't look young anymore. But there is actually no more ways to address to them. Sometimes I envy english-speaking persons as they don't need to think too much about addresses and only have "you" not "ты/вы". It really simplifies communication.

Not necessarily. I could refer to you as a person, like saying you're a good person, even if I am aware of your gender. I suppose Человек is a perfect equivalent in Russian, whenever you would use "person" in English.

But че sounds like чи even though it is not immediately before the stress(which you said it's on the last syllabus), therefore е should sound like "yuh" according to russianforeveryone, what am I missing?

Probably because of the "the" - because "этот" is usually translated with "this" or "that", which is more specific than "the". If I wanted to say "Where is the person?", my first instinct would be to translate it as "Где человек?"